1. Field of the Invention
The present invention relates to a polishing sheet and a polishing work method, and in particular relates to a polishing sheet having an elastic plastic foam sheet in which fine particles are contained and a polishing work method using the polishing sheet.
2. Description of Related Art
Conventionally, a polishing sheet is used to perform flattening work on a material required for flatness at a high accuracy, such as an optical material such as a lens, a plane parallel plate, a reflecting mirror or the like, a base plate for a hard disc, a silicon wafer, a liquid crystal glass or the like. For example, as a polishing sheet used for accurate flattening work, e.g. for the silicon wafer or the like, a polishing sheet of a nonwoven type obtained by drying a nonwoven sheet after the nonwoven sheet is impregnated with a resin liquid is known (refer to JP05-8178A publication). However, since the polishing sheet of this type has flexibility, there are drawbacks in that a polishing efficiency is low and a roll-off, i.e. peripheral edge portions of a material to be polished are polished more than a central portion thereof, occurs easily. In order to improve the polishing efficiency, there has been disclosed a polishing sheet having an independent foam structure obtained by hollow fine spherical bodies or the like in a plastic material with a high hardness (for example, refer to JP08-500622A publication). However, because a polishing sheet of this type has a high hardness, for example, when the sheet is used in polishing work for an aluminum base plate for a hard disc, there is a drawback in that defects may occur at a surface of the aluminum base plate.
On the other hand, a polishing sheet obtained by pasting a film with a high hardness or the like to an elastic plastic foam of a suede type with a foam structure produced by wet film forming process has been disclosed (for example, refer to JP10-249709A publication). An elastic plastic foam is generally produced by the wet film forming process, and relatively large cells extending in a direction approximately perpendicular to a polishing face are formed in the interior of the elastic plastic foam. Further, elastic plastic exists between adjacent cells in a manner of a partition wall, and relatively small fine foams whose sizes have not been controlled are formed in the partition wall.
Further, a polishing sheet for a silicon wafer where fine particles of barium carbonate are contained in an elastic plastic foam as abrasive particles has been disclosed (for example, refer to JP2001-1270A publication).
However, since the polishing sheet disclosed in the JP10-249709A publication has an elastic plastic foam sheet as a base sheet, there are problems about the polishing efficiency and the roll-off like that in the JP05-8178A publication. Further, when the polishing sheet of the JP2001-1270 publication is used in polishing work for an aluminum base plate, there is such a problem that defects may occur at a surface of the aluminum base plate by the fine particles contained in the polishing sheet. Accordingly, there is a need for development of a polishing sheet which can secure flatness of a material to be polished and has a high polishing efficiency, and which does not cause defects at a surface of the material to be polished, such as an aluminum base plate or the like.